1. Field of the Invention
This invention relates generally to switch-mode power converters and more particularly it provides simple a drive circuit and an enable function with isolation and having high performance in full-bridge topologies using synchronous rectification.
2. Background Discussion
A switch-mode power converter is a circuit that uses an inductor, a transformer, or a capacitor, or some combination, as energy storage elements to transfer energy from an input source to an output load in discrete pulses. Additional circuitry is added to maintain a constant voltage within the load limits of the circuit. The basic circuit can be configured to step up (boost), step down (buck), or invert output voltage with respect to input voltage. Using a transformer allows the output voltage to be electrically isolated from the input voltage.
Switch-mode converters have changed very little over the past 15 years, most using Schottky diodes to rectify their output. However, newer challenges in the industry for dc/dc power supply designers demand lower voltages required by digital circuits, and also higher frequencies. Since converters using Schottky diodes for rectification experience a large forward voltage drop relative to the output voltage, their efficiency is generally relatively low. Lower efficiencies result in more dissipated heat that has to be removed using a heat sink, which takes up space. A dramatic increase in converter efficiency can be accomplished by replacing the Schottky diodes with xe2x80x9csynchronous rectifiersxe2x80x9d realized in practice with MOSFET transistors. Synchronous rectifiers are not new, but they have previously been too expensive to justify, primarily due to high xe2x80x9conxe2x80x9d resistance. However, as costs fall and performance improves, synchronous rectifiers have quickly become a viable component, especially for low voltage converters.
Using self-driven synchronous rectifiers in various converter topologies is very attractive and popular because there is no need for additional isolation between drive signals. It has the advantage of simplicity. However, it has the disadvantage of cross conduction between synchronous rectifiers and primary side switches, as well as reverse recovery current of the parasitic anti-parallel diode of the MOSFETs used for synchronous rectification. In order to minimize these shoot-through currents, an inductance (or saturable inductor) is usually placed in series with the synchronous rectifier. While this may be a solution for lower switching frequencies, for example, 100 kHz-200 kHz, it is not suitable for higher switching frequencies (200 kHz and above). Especially at switching frequencies of 300-400 kHz this is not an optimum solution. The reason for this is that increased inductance in series with a synchronous rectifier reduces the effective duty cycle on the secondary side of the power transformer due to slower di/dt of the secondary current. As a result, more voltage headroom is required in the power transformer, implying a smaller effective turns ratio and lower efficiency.
A second reason why self-driven synchronous rectification is not suitable for higher switching frequencies is the potential loss due to reverse recovery current in the body diode of the synchronous rectifiers (MOSFETs) and increased turn-on current in the primary side switches (usually MOSFETs).
A third reason why self driven synchronous rectifiers have not been a preferred solution is that the drive voltage, being derived from a power transformer, depends on input voltage and therefore could vary significantly (200% to 300%). As a consequence, power consumption of the drive circuit, which varies exponentially with input voltage, can vary even more (400% to 900%) and decrease overall converter efficiency.
A much more preferred solution is to use direct drive to power synchronous rectifiers with well-controlled timing between drive signals for the main switches (primary side) and synchronous rectifiers (secondary side). This solution thus allows for very efficient operation of the synchronous rectifiers even at high switching frequencies. Yet another benefit of direct driven synchronous rectifiers is that the drive voltage (gate to source) is constant and independent of input voltage, which further improves efficiency over a wide input voltage range.
It is necessary to provide delays between drive signals for primary side switches and secondary side switches in order to avoid cross conduction (simultaneous conduction which would result in a short circuit). When power converters are operated at lower switching frequencies (for example, 100 kHz), cross conduction of the switches can be acceptable since the percentage of the time during which cross conduction occurs relative to the switching period is small (typically 40 ns/10 xcexcs). Also, a transformer designed to operate at lower frequencies will have a larger leakage inductance, which will reduce cross conduction currents. In the case of higher switching frequencies (above 100 kHz), cross-conduction becomes more unacceptable (40 ns/2 xcexcs for a 500 kHz switching frequency). Also for higher switching frequencies, the leakage inductance in the transformer as well as in the whole power stage should be minimized for higher efficiency. Consequently, currents due to cross conduction time can become significant and degrade overall converter efficiency and increase heating of the power components significantly.
In an embodiment of the invention, one drive transformer is used for providing appropriate delays as well as providing power for driving primary switches, particularly high side switches in a full-bridge topology. The leakage inductance of the drive transformer is used to delay turn-on of the main switches (primary side) while turn-off is with no significant delay. The number of windings on the drive transformer is minimized to four, when the control circuit is referenced to the output of the converter, and minimized to five when the control circuit is referenced to the input of the converter. In the full-bridge converter, having the control circuit referenced to the output of the converter, four windings are for: (1) the control and drive circuit (pulse width modulated (PWM) type, for example) signal referenced to the output and providing proper waveforms for driving synchronous rectifiers; (2) driving two bottom primary side switches; (3) driving one top primary side switch; and (4) driving second top primary side switch. If the control circuit is referenced to the input of the converter, there are five windings for: (1) the control and drive circuit signal referenced to the input of the converter; (2) providing proper waveforms for driving synchronous rectifiers; (3) driving one top primary side switch; (4) driving a second top primary side switch; and (5) driving two bottom primary side switches. It is an additional object of the invention to provide means to enable/disable the module due to a condition sensed on either the input or the output side via a controller or protection circuit located on either the input or the output side of the converter.